The present invention relates to an advanced rotation system used in x-ray generation, and more particularly, to a high efficiency motor employed in such system.
Typically, an x-ray beam generating device, referred to as an x-ray tube, comprises opposed electrodes enclosed within a cylindrical vacuum vessel. The vacuum vessel is typically fabricated from glass or metal, such as stainless steel, copper or a copper alloy. The electrodes comprise a rotating disc-shaped anode assembly and a cathode assembly that is positioned at some distance opposite from the target track of the anode assembly. The target track, or impact zone, of the anode is generally fabricated from a refractory metal with a high atomic number, such as tungsten or tungsten alloy. Further, to accelerate the electrons, a typical voltage difference of 60 kV to 150 kV is maintained between the cathode and anode assemblies. The hot cathode filament emits thermal electrons that are accelerated across the potential difference, impacting the target zone of the anode at a high velocity. A small fraction of the kinetic energy of the electrons is converted to high energy electromagnetic radiation, or x-rays, while the balance is contained in back scattered electrons or converted to heat. The x-rays are emitted in all directions, emanating from the focal spot, and may be directed out of the vacuum vessel. In an x-ray tube having a metal vacuum vessel, for example, an x-ray transmissive window is fabricated into the metal vacuum vessel to allow the x-ray beam to exit at a desired location. After exiting the vacuum vessel, the x-rays are directed to penetrate an object, such as human anatomical parts for medical examination and diagnostic procedures. The x-rays transmitted through the object are intercepted by a detector and image is formed of the internal anatomy. Further, industrial x-ray tubes may be used, for example, to inspect metal parts for cracks or to inspect the contents of luggage at airports.
With the increase in x-ray tube size and power dissipation capabilities there has been an associated evolution in motor technology employed for the rotation of the x-ray tube anode assembly. Currently, the most powerful and advanced rotation system used in x-ray tube technology is an induction motor that leverages an anode potential stator which results in a motor with near traditional magnetic gap spacing of approximately 1.5 mm to 2.0 mm between the stationary and rotating components. The stationary members include the stator frame and windings, while the rotating members include the rotor assembly encompassing a magnetic yoke and copper current carrying conductors. The near traditional magnetic gap spacing allows the use of commercially available control systems and delivers performance parameters typical of poly-phase induction motor machines.
One application for an induction motor having this near traditional magnetic gap spacing, termed a High Efficiency Motor (HEM), is in x-ray tubes with metal vacuum containment enclosures utilized on high performance diagnostic systems. FIG. 1 shows the application of the HEM 10 in a typical high voltage sub-system 12 for x-ray generation. At first glance, the HEM 10 is basically a three-phase induction motor that includes a stator 14, which comprises a stator frame and three-phase windings, and a rotor 16, which is electrically connected and physically attached to the rotating disc-shaped anode assembly 18 to perform the rotation. A metal vacuum enclosure, or frame 20 encloses both the rotor and the anode assembly. The HEM 10, however, is unique in that the stator 14 and rotor 16 are in very close proximity to each other, unlike most traditional x-ray tubes. This is made possible by making the metal frame 20 thin enough to support near traditional gaps, while maintaining high structural stability and strength. It is this close proximity of the stator to the rotor that produces the high performance aspects of the HEM 10, which can deliver 2.0 HP in the acceleration phase and employ dynamic braking schemes because of the relatively good coupling of the rotor to the stator. It should be noted that vacuum frame 20 can also be made of glass that is thick enough to withstand the vacuum pressure while sufficiently thin to afford the desired near traditional gap.
Because of the power requirement for x-ray generation, the anode assembly 18 is supplied with a high potential, for example, of about 70,000 volts by a high voltage anode supply 22 via a high voltage (HV) cable 24. Since the rotor 16 is electrically connected to the anode assembly 18, the rotor is fed with the same high potential. The close proximity of stator 14 to rotor 16 also requires that the stator itself be running at such high potential to achieve the tight gap. In other words, the three phase windings of the stator 14 are referenced to the anode potential of about 70,000 volts instead of ground. These windings, however, are being powered by a motor controller 26 that is ground referenced with three-phase voltage outputs 28. Due to the difference in potential referencing, an isolation transformer 30 is thus used to electrically insulate and protect the stator assembly 14 and the motor controller 26 from each other. The isolation transformer 30 is a one-to-one transformer, in this case a delta-to-delta transformer, with one side connected to the motor controller 26 referenced at ground, and the other side connected to the stator assembly 14 referenced at the high voltage anode supply 22 of about 70,000 volts.
The major disadvantage to the above prior art approach of the HEM is the requirement for an isolation transformer 30 to electrically insulate the stator assembly 14, which is referenced to the operating potential of the x-ray tube, and the output of the motor controller 26, which is three-phase ground referenced. The necessary, separate isolation transformer 30 requires additional hardware be added to the high voltage sub-system 12 for x-ray generation, which negatively impacts cost, size and reliability. There is thus a need for eliminating the separate isolation transformer while retaining its isolation function to electrically insulate and protect the motor 10 and the motor controller 26 from each other. Additionally, the separate isolation transformer between ground and high potential in the prior art system is disadvantageous because of their excessive power consumption and heat generation of the isolation transformer.